Many different devices employ fluid level sensors to detect fluid levels in reservoirs. The detection may trigger automated fluid replenishment, be included in status reports, or may only report when levels get low and need replenishing, as examples. One such application involves sensing the levels in ink reservoirs used in printing systems. While this is one example, the ink sensing application demonstrates some of the issues with current sensors.
One issue that occurs in this area results from the wide array of print systems that use a similarly wide array of inks. Inks may consist of many different materials and many different combinations of these materials, all with differing characteristics. For example, different inks have different conductivity properties. Conductive-based ink level sensors have worked well when designed for specific inks used in specific products. For example, some of these inks have an electrical conductivity range of 10:1 across manufacturing lots and use conditions. However, in the wider uses covering multiple ink chemistries, the conductivity range may exceed 1000000:1.
Some products use self-heating thermistor probes. However, these may show false empty readings with heated incoming ink. The heat from the probe may also degrade certain UV sensitive inks. More than likely, one type of thermistor probe will not work across ranges of ink types and characteristics.
Other products employ optical ink level sense probes. These probes may degrade at the high temperatures used for some meltable inks. They also may get coated with ink pigments, and generally have poor signal differentiation between full and empty signals on highly loaded white pigmented inks.
Still other products use capacitive ink level sensing. This works well for non-conductive through moderately-conductive inks. Highly conductive inks, especially in metal reservoirs or tanks, can cause problems. Thin films of ink coating the probe can cause false full readings. False empty readings can also occur if ink shorts the probe signals to the reservoir housing.
The ability to provide and use a single type of sensor that can operate in different types of inks across a wide range of temperatures would be useful.